The present invention relates to method and apparatus for shaping rigid objects by grinding or polishing; more particularly, to method and apparatus for finishing by impingement of a fluid jet onto a rigid object, such as a glass or ceramic lens or a metal object; and most particularly, to method and apparatus for impinging a fluid jet, such as an air jet or a water jet, onto an abrasive-bearing liquid film in contact with a surface of an object to be shaped by removal of material therefrom.
It is known to use abrasive fluids to shape, finish, and polish objects, especially optical elements such as lenses and mirrors. See, for example, U.S. Pat. No. 5,951,369, xe2x80x9cSystem for Magnetorheological Finishing of Substrates,xe2x80x9d issued Sep. 14, 1999 to Kordonsky et al. Also, see xe2x80x9cPrinciples of Abrasive Water Jet Machining,xe2x80x9d byA Momber and R. Kovacevic, published by Springer-Verlag London, Ltd. (1998), especially pp. 328-330. As used herein, the term xe2x80x9cgrindingxe2x80x9d means relatively rapid and coarse removal of material to change the global shape of an object; the term xe2x80x9cpolishingxe2x80x9d means relatively slow and fine removal of material to reduce the micro-roughness of a surface already formed as by grinding or other gross process. As used herein, all removal processes, including grinding, polishing, and machining, whereby a surface is shaped, are referred to collectively as xe2x80x9cfinishing.xe2x80x9d
In the known art of jet finishing, a liquid slurry containing abrasive particles suspended in a liquid carrier medium is impinged at high velocity against a substrate surface to be finished. See, for example, U.S. Pat. No. 5,700,181, issued Dec. 23, 1997 to Hashish et al. The abrasive particles are sufficiently energetic to break loose particles of the substrate by mechanical attack, which substrate particles are then carried away by the slurry. Such finishing may be considered a form of mechanical grinding.
Jet impingement finishing as practiced in the known art has several serious shortcomings.
For example, the abrasive slurry typically must be maintained in a mixed state in a reservoir. Particulate abrasives typically are prone to rapid settling and thus require active mixing.
Further, the abrasive slurry must be pumped by a special abrasion-resistant pump through an abrasion-resistant delivery system and nozzle. Useful lifetimes of nozzles are known to be relatively short.
Still further, the abrasive particles are prone to settling in the slurry delivery system, thereby causing blockages and stopping flow.
Still further, known finishing systems are not well-suited to finishing very small objects or surfaces, for example, the ends of fiber-optic strands. The minimum diameter of the jet is limited by the size of the abrasive particles, or clumps thereof, which must be delivered through the nozzle. Very small diameter nozzles are readily clogged, and high pumping pressures are required to maintain high-velocity flow. Thus there is a practical lower limit on the size of substrates which may be finished by prior art apparatus and methods.
What is needed is a method and apparatus for fluid jet surface finishing of micro- and nano-sized objects.
It is a principal object of the invention to provide an improved method and apparatus for jet finishing wherein the minimum size of the surface to be finished is not limited by the size of the abrasive particles nor the diameter of a nozzle for impinging an abrasive jet thereupon. It is further object of the invention to provide an improved method and apparatus for jet finishing by a nozzle wherein the nozzle cannot be plugged by abrasive particles.
It is a still further object of the invention to provide an improved method and apparatus for jet finishing wherein both grinding and polishing may be performed by adjustment of a given finishing apparatus.
It is a still further object of the invention to provide an improved method and apparatus for inexpensively forming microlenses.
Briefly described, a method and apparatus for finishing of a substrate surface in accordance with the invention includes means for covering the surface with a liquid slurry containing abrasive particles and means for impinging a jet of fluid, preferably air or water, against the slurry to accelerate the particles and induce formation of a high-shear work zone on the substrate surface wherein portions of the substrate are lifted and removed by the slurry to alter the shape of the substrate surface towards a predetermined shape and/or smoothness. The surface may be covered, for example, by cascading a flowing layer of slurry over it, or by impinging slurry onto the work zone, or by immersing the substrate in a pool of the slurry, or the like. The jet is provided, for example, by a tubular nozzle having an exit orifice which may be precisely located at a predetermined distance from the surface to be finished. A coarse removal function may be provided by establishing the exit orifice at a first distance from the substrate surface, and a fine removal or polishing function may be provided by placing the exit orifice at a second and closer distance from the substrate surface. Further, the areal shape of the removal function may be varied by varying the distance and angle between the nozzle and the substrate; and at certain spacings, the function is radially bimodal, permitting simple and inexpensive formation of curved surfaces such as microlenses. The nozzle may be oriented such that the axis of the jet forms a predetermined angle with the surface to be finished, between 0xc2x0 and 90xc2x0. The exit orifice may be immersed in the slurry or may be disposed in space above the free surface of the slurry. The slurry may be aqueous or otherwise. Preferably, the slurry has a viscosity somewhat higher than that of water, such that a substantial rate of surface shear is induced in the slurry by the impingement of the jet. Preferably, the substrate and/or the nozzle may be controllably moved past one another to obtain the desired contour or smoothness of the substrate surface.